silica nanocomposites

G Model

ARTICLE IN PRESS

JECS-10388; No. of Pages 8

Journal of the European Ceramic Society xxx (2015) xxx–xxx

Contents lists available at www.sciencedirect.com

Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc

Facile sol–gel synthesis of reduced graphene oxide/silica nanocomposites Cornelia Hintze a,1 , Koji Morita b , Ralf Riedel a , Emanuel Ionescu a,c,∗ , Gabriela Mera a,∗ a b c

Technische Universität Darmstadt, Institut für Materialwissenschaft, Jovanka-Bontschits-Strasse 2, D-64287 Darmstadt, Germany National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan Universität zu Köln, Department Chemie, Institut für Anorganische Chemie, Greinstrasse 6, D-50939 Köln, Germany

a r t i c l e

i n f o

Article history: Received 14 August 2015 Received in revised form 18 November 2015 Accepted 24 November 2015 Available online xxx Keywords: Silica–graphene nanocomposites Graphene Sol–gel synthesis Silicon oxycarbide

a b s t r a c t Novel silica-containing nanocomposites with a controlled carbon phase based on high quality reduced graphene oxide (rGO) were prepared via a cheap and facile sol–gel method, followed by pyrolysis in inert gas atmosphere. The resulting nanocomposite materials have an intimate bonding between rGO of low defect density and a partially crystalline silica matrix, as shown by Raman, FTIR and HRTEM studies. This processing route allows a straight-forward control of the content of the carbon phase and an excellent dispersion thereof within the silica matrix. © 2015 Elsevier Ltd. All rights reserved.

1. Introduction Polymer-derived ceramics (PDCs) route offers the possibility to synthesize novel ternary ceramic systems of different composition and microstructures by the controlled thermolysis of suitable precursors with a variety of chemical structures. An important class of ternary PDCs is represented by silicon oxycarbide (SiOC) ceramics. This class has attracted a lot of attention in the past 40 years due to its excellent resistance to crystallization and creep deformation at high temperatures as well as its facile synthesis [1]. Usually, polysiloxane polymers have been used as precursors for SiOC ceramics [1], which are amorphous, glassy materials consisting of mixed-bonds SiOx C4−x tetrahedra and some amount of segregated carbon (or free carbon). The free carbon phase was derived from the thermolysis of organic substituents bonded at silicon in the polymer state. This microstructure was extensively investigated during the years and several microstructural models were proposed [2]. A special attention was offered to the free carbon phase, which was found to strongly influence the thermal stability against crystallization and decomposition of these ceram-

∗ Corresponding authors. E-mail addresses: [email protected] (E. Ionescu), [email protected] (G. Mera). 1 Current address: Karlsruhe Institut für Technologie, Institut für Festkörperphysik, 76121 Karlsruhe.

ics, as well as their functional and mechanical properties. It was found that the carbon phase is composed of randomly oriented fine graphene layers in a matrix of amorphous SiOC. It was postulated that the orientation, concentration as well as interface bonding of this graphene phase with the matrix should be the key for understanding the exceptional properties of this class of ceramics [1,2]. Thus, PDCs are intrinsically complex nanostructured systems, which may undertake significant microstructural changes when exposed to high temperatures. Simple model systems such as the graphene/silica nanocomposites in the present study can help to understand the structure and the interface bonding in silicon oxycarbides, features which are controlled by the concentration and dispersion of the carbon phase in the ceramic matrix. However, on should mention that the model system presented in this work have also some limitations. Thus, the prepared monolithic rGO/silica samples were hot-pressed at high temperatures (i.e., 1600 ◦ C) and consequently showed some crystallization of the silica phase. This is not the case for silicon oxycarbides, which were shown to keep their amorphous nature up to their decomposition temperature. Nevertheless, the rGO/silica samples presented in this work are considered to exhibit one main common feature with single-phase and phase-separated silicon oxycarbides, i.e., the fate of the carbon phase and its bonding to the matrix. Ever since first synthesized [3], the two-dimensional nature [4] and the exceptional electronic [5–13] and mechanical properties [11] of graphene have gained a lot of interest. As pristine

http://dx.doi.org/10.1016/j.jeurceramsoc.2015.11.033 0955-2219/© 2015 Elsevier Ltd. All rights reserved.

Please cite this article in press as: C. Hintze, et al., Facile sol–gel synthesis of reduced graphene oxide/silica nanocomposites, J Eur Ceram Soc (2015), http://dx.doi.org/10.1016/j.jeurceramsoc.2015.11.033

G Model JECS-10388; No. of Pages 8

ARTICLE IN PRESS C. Hintze et al. / Journal of the European Ceramic Society xxx (2015) xxx–xxx

2

graphene sheets with large lateral dimensions are insoluble in all common solvents [14], graphene oxide (GO) is the material of choice for the preparation of nanocomposites containing well dispersed graphene. GO contains reactive groups as carboxyl, hydroxyl and ketone groups at the edges as well as hydroxy and epoxy groups within the plane [15,11,16,17]. The presence of hydroxy groups in GO offers the possibility to disperse it in water and for using GO for example in condensation reactions [17]. This feature makes GO an interesting and suitable precursor for sol–gel reactions in aqueous media. The electrical conductivity of GO is relatively low and relies on the disruption of the ␲-network. This issue can be partially solved via chemical [13,18–20] (e.g., upon exposure to hydrazine monohydrate [19,12]), or thermal reduction processes [21–23]. The functional groups in GO were previously used to synthesize polymer nanocomposites composed of graphene and silica [24–27,5,28–30] or silicone matrices [31,32]. Usually, an appropriate functionalization of GO was performed in a first step [6,7,18,33–35]. In order to study the effect of the segregated carbon phase (i.e., content, organization, interfaces etc.), novel model systems for assessing the bonding of the carbon phase in SiOC ceramics were produced upon the pyrolysis of highly cross-linked gels composed of single-/few-layer reduced graphene oxide and silica. The gelation of rGO with TMOS offers the possibility to incorporate the graphene phase in a controlled way in the silica matrix. No supplemental chemical functionalization of the GO phase was necessary to provide good homogeneity of the nanocomposites. After pyrolysis of obtained gels containing different concentrations of graphene oxide, nanocomposite materials consisting of a high-quality rGO homogeneously dispersed within a silica-rich matrix was obtained.

2. Experimental procedure All chemicals were obtained from Sigma–Aldrich and used asreceived. GO was synthesized using a modified Hummer’s method based on reference [36]. Thus, 3 g of graphite were dispersed in a mixture of sulfuric (H2 SO4 ) and phosphoric (H3 PO4 ) acids (volume ratio of H2 SO4 :H3 PO4 = 8:1). 18 g of potassium permanganate (KMnO4 ) was subsequently added [37] and the mixture was refluxed at 60 ◦ C for 12 h and poured over 400 ml ice mixed with 5 ml H2 O2 (30%). The reaction mixture was centrifuged for 3 h at 6000 rpm in water, hydrochloric acid (HCl) (30%) and twice in ethanol. After each centrifugation step, only the finest solids were kept, whereas the coarse, non-exfoliated GO was removed. The obtained GO-ethanol dispersion was then dried at 40 ◦ C under vacuum before re-dispersion in water. The nanocomposites were prepared using different contents of GO, i.e., 0.1, 0.5, 1, 2, 3, 4, 5, 6, 10 and 20 wt% with respect to TMOS. The resulting samples are referred to as SG 0.1, SG0.5, SG1, SG2, SG3, SG4, SG5, SG6, SG10 and SG20, respectively. The GO-silica sols were prepared upon adding ethanol and TMOS to a dispersion of GO in water. The water content was adjusted to maintain a molar ratio of 1:1:0.1 between water, ethanol and TMOS [27]. The gelation process was performed at 60 ◦ C and a pH value of 9 (controlled upon ammonia addition) [29,38]. The obtained wet gels were placed into a drying cabinet at 60 ◦ C and allowed to age for 5 days. The resulting xerogels were first dried under vacuum, at room temperature, and then reacted for 24 h with hydrazine hydrate [18,13,39] .The subsequent pyrolysis processes were conducted at 900 ◦ C under argon atmosphere. The pyrolyzed rGO-silica powders were pressed in graphite dies into monoliths (KCE hot press HP W 150/200, argon atmosphere, 30 MPa, 1600 ◦ C, 30 min.). In order to avoid carbon diffusion into the samples from the graphite dies, their walls were covered with boron nitride.

FTIR spectroscopy was performed on a Bruker Vertex 70/70 v spectrometer for the prepared powders; whereas the monolithic samples were measured in Attenuated Total Reflection (ATR) mode with an Ivan 670 IR Spectrometer (Varian, USA). Micro-Raman spectra were recorded using a Horiba HR800 micro-Raman spectrometer (Horiba Jobin Yvon, Bensheim, Germany) equipped with an Ar+ (514.5 nm) laser. The excitation line has its own interference filter (to filter out the plasma emission) and a Raman notch filter (for laser light rejection). The measurements were performed with a grating of 600 g mm−1 and a confocal microscope (magnification 50×, NA 0.5) with a 100 ␮m aperture, giving a resolution of 2–4 ␮m. The laser power (ca. 20 mW) on the sample was attenuated in the range of 2 mW–20 ␮W using neutral density (ND) filters. UV–vis spectrometry was performed on a Lambda900 spectrometer (PerkinElmer, USA). The band gap of the materials was estimated from the recorded UV–vis spectra using the Tauc relationship [40]. High resolution scanning electron microscopy was performed on a HRSEM, XL 30 FEG machine (Philips, Hamburg, Germany) equipped with Energy Dispersive X-ray Spectroscopy (EDX: EDAX Genesis, EDAX, USA). The high resolution transmission electron microscopy studies were done on a JEOL-2100F microscope (200 kV). X-ray diffraction measurements were performed on a Bruker D8 Advance diffractometer (Bruker, USA). and on a Seifert PTS 3003 diffractometer for powders and monoliths, respectively. Selected samples were analyzed additionally by means of atomic force microscopy (AFM, MFP 3D, Asylum Research, Oxford Instruments, USA) and conductive AFM (Orca) The electrical properties of the samples were assessed using four point resistance measurements (Keithley 2400 devices) and impedance spectroscopy (IS) was done with a a Solartron SI 1287 Impedance Analyzer (Solartron Analytical, AMETEK, USA).

3. Results and discussion 3.1. GO synthesis and its conversion into rGO Prior to the incorporation of GO into the silica sol, the quality of the prepared GO was assessed by means of Raman and FTIR spectroscopy. In Fig. 1(a), the Raman spectra of GO and the reduced material, rGO (obtained upon hydrazine exposition of GO followed by pyrolysis in argon) are shown. Due to the high degree of functionalization of GO, the single, well-defined 2D mode at 2704 cm−1 which is characteristic for graphene, is broad and of low intensity and it partially overlap with the D + D and the 2D bands to form a modulated bump from 2300–3400 cm−1 , indicating either very small size of the graphene particles or a high density of defects [33,34]. Upon pyrolysis, the integrated intensity of the D band decreases with respect to the integrated intensity of the G band from 3.4 to 1.5. This reduction in the I(D)/I(G) ratio is consistent with a less defective sample after pyrolysis. The FWHM of the D band decreases significantly after pyrolysis (almost by 50%), confirming that the defect density in the graphene sheets is reduced as a consequence of the removal of functional groups from GO during pyrolysis [21]. Additionally one can note that the 2D band does not shift upon pyrolysis. A shift of 2D is considered to rely on mechanical strain [41,42], which is consequently absent here. The G band shifts to lower wavenumbers (from 1602 cm−1 in GO to 1585 cm−1 in rGO). This is thought to rely on the reduction of GO (though the shift of the G band is less sensitive to intrinsic strain than the 2D band [21,41–47]). The FTIR spectrum of GO (Fig. 1(b)) shows an absorption band at about 3400 cm−1 which was assigned to hydroxy groups

Please cite this article in press as: C. Hintze, et al., Facile sol–gel synthesis of reduced graphene oxide/silica nanocomposites, J Eur Ceram Soc (2015), http://dx.doi.org/10.1016/j.jeurceramsoc.2015.11.033

G Model JECS-10388; No. of Pages 8

ARTICLE IN PRESS C. Hintze et al. / Journal of the European Ceramic Society xxx (2015) xxx–xxx

3

Fig. 1. Raman spectra, (a) FTIR spectra (b) of as-prepared GO and of rGO obtained upon thermal reduction of GO with hydrazine followed by pyrolysis in argon atmosphere; (c) XRD pattern of the as-prepared GO.

present in the sample. Additionally, a band at 1230 cm−1 was also assigned to C OH groups in GO. These functional groups are of particular importance, as they were used to provide covalent bonding between the GO and the silica-based sol–gel matrix via condensation reactions. Interestingly, the hydroxy groups seem to not fully be removed upon pyrolysis; this is rather unusual, as hydroxyl groups are not very heat resistant. Vibrations from saturated CHx end groups are observed close to 3000 cm−1 [20,34]; whereas the strong vibration at 1735 cm−1 was assigned to carboxy and carbonyl groups [7,30,33,41,48–50] and the broad band around 1100 cm−1 relates to C O C stretching vibrations belonging to both ether and epoxy type functional groups [20,30,33,41,47,51–53], which are typical in GO materials obtained from Hummer’s method [11,16,54]. The pyrolysis of GO leads to a blue-shift of most bands, which indicates the reestablishment of ␲-bonds upon the reduction of GO [52]. This is supported by the C C vibration, whose intensity compared to the intensity of the epoxy vibrations has increased from a mere shoulder. Upon pyrolysis, a number of oxygen-containing groups such as carboxylic substituents are reduced, however the C O C bond proves strong enough to be present even after pyrolysis. The XRD in Fig. 1(c) shows (0 0 1) reflection at 2 = 10.36◦ , which corresponds to a lattice spacing of 8.53 Å and is similar to values of 8.3–8.7 Å reported for GO samples produced by the Hummer’s method [20,25,32,48,54]. AFM analysis (not shown here) indicates

topography steps being present in the GO samples with heights of 1.6 and 2.4 nm. These values correspond to bi- and tri-layer aggregates with a inter-layer distance of 0.8 nm. In literature, the interlayer spacing of 0.8 nm was reported for GO intercalated with water molecules [39,55]. In contrast, our XRD data yields a lattice spacing of 0.85 nm, which is slightly larger and may be due to the presence of diethyl ether between the graphene layers of GO. 3.2. Synthesis of the graphene–silica nanocomposite (GSN) powders The nanocomposites were prepared via sol–gel technique upon hydrolysis and condensation of basic TMOS solutions in water/ethanol mixtures to yield homogeneous gels. Fig. 2 shows the SEM micrographs of the as prepared rGO–silica nanocomposite as obtained from the pyrolysis. The edges of graphene sheets are clearly visible in Fig. 2(a) and in Fig. 2(b) silica nanoparticles of ca. 35 nm in size can be observed. High-resolution transmission electron microscopy (HR-TEM) investigation of the amorphous graphene–silica nanocomposites (Fig. 3) confirmed the formation of homogeneous nanocomposite materials. Furthermore, the carbon phase was shown to consist mostly of few-layer rGO. Here, an average inter-layer distance of 0.382 nm was measured, which is only slightly larger than the Dspacing of graphite [54] and agrees well with the inter-layer spacing of twisted few-layer graphene-based materials [57]. This finding

Please cite this article in press as: C. Hintze, et al., Facile sol–gel synthesis of reduced graphene oxide/silica nanocomposites, J Eur Ceram Soc (2015), http://dx.doi.org/10.1016/j.jeurceramsoc.2015.11.033

G Model JECS-10388; No. of Pages 8 4

ARTICLE IN PRESS C. Hintze et al. / Journal of the European Ceramic Society xxx (2015) xxx–xxx

Fig. 2. SEM micrographs of a rGO-silica nanocomposite: (a) low magnification micrograph showing edges of rGO, which is embedded within silica; (b) high magnification micrograph showing the silica matrix consisting of nanoparticles with an average size of ca. 35 nm (the particle size was estimated upon using a image analysis software developed in house, Lince 2.4.2e, based on the linear intercept method).

Fig. 3. HRTEM at different magnifications (a and b) and the selected area electron diffraction (c) of a rGO-silica nanocomposite.

indicates an efficient reduction of GO at 900 ◦ C (note that the interlayer spacing of GO prior to reduction is 0.85 nm and thus more than 2 times larger, as discussed above). As the flake edges indicated by arrows in Fig. 3(a) appear reasonably sharp, thermal reduction is also confirmed at the edges [22]. Additionally, no significant amount of residual functional groups is assumed to be present in the rGO phase, since this would increase the interlayer spacing significantly more than 0.4 Å. The selected area electron diffraction (SAED) pattern of diffuse rings (Fig. 3(c)) indicates the amorphous nature of the silica matrix as well as the absence of long-range order in rGO. Furthermore, the wrinkled microstructure of the graphene sheets in Fig. 3(b), confirms that the rGO phase is consisting of only few graphene layers [56] without long-range stacking. Fig. 4 shows the FTIR spectra of the sample SG6 as wet gel, after hydrazine treatment and as-pyrolyzed. The gel sample shows an absorption band at 1080 cm−1 which is characteristic for orthosilicate-based gels [7,26,28,41,52,53] and corresponds to Si O Si/Si O C bonds. All samples show an absorption band at about 1623 cm−1 , which relates to the bending vibration of residual water molecules [7,11,48,58]. The band at 1127 cm−1 (related to epoxy stretching vibrations [33,30]), which is present in the wet gel disappears upon hydrazine treatment. Additionally, the band related to C OH groups (1208 cm−1 in the gel and ∼1181 cm−1 after hydrazine treatment) disappears upon pyrolysis, indicating an effective thermal reduction of the GO at 900 ◦ C. Although the reaction between GO and silica cannot be directly supported by the FTIR data, there is some indirect evidence of a successful condensation process between the orthosilicate gel and the GO phase. Thus, the C OH related band at 1230 cm−1 [59] which is present in GO (Fig. 1(b)) is not present in the GO-containing gel samples. Thus, it is considered that the hydroxyl functional groups present in the GO phase react with the silanol (Si OH) and methoxy

Fig. 4. FTIR of SG6 as wet gel (a), after hydrazine treatment (b) and after pyrolysis in argon (c).

(Si O CH3 ) groups and provide a covalent bonding between the GO phase and the silica-based gel. Further evidence of the chemical bonding between GO and silica can be deduced from the Lorentzian fitting of the bands of the Raman spectra, as presented in Fig. 5. As GO gets reduced (chemically and thermally), the FWHM of the D and 2D bands decreases, confirming the reduction reaction and the decrease of the number of defects. Furthermore, the relative intensity of the D** band (i.e., I(D**)/I(G), inset of Fig. 8), which is related to disordered and amorphous carbons, is decreasing from 0.44 for the as-gelled sample to 0.26 after pyrolysis.

Please cite this article in press as: C. Hintze, et al., Facile sol–gel synthesis of reduced graphene oxide/silica nanocomposites, J Eur Ceram Soc (2015), http://dx.doi.org/10.1016/j.jeurceramsoc.2015.11.033

G Model JECS-10388; No. of Pages 8

ARTICLE IN PRESS C. Hintze et al. / Journal of the European Ceramic Society xxx (2015) xxx–xxx

5

of the FWHM is more significant for the pure GO sample as for the GO/silica nanocomposites. This is considered as further support of a successful reaction between the GO and TMOS during the synthesis of the GO/silica gel sample. In conclusion, the Raman spectroscopic data reveal that the GO phase binds covalently to the silica gel matrix via Si O C linkages, thus leading to the formation of highly homogeneous GO/silica nanocomposites.

3.3. Fabrication of monolithic rGO/silica nanocomposites upon hot-pressing

Fig. 5. Comparison of micro-Raman spectra for SG3-based samples: wet gel (a), after hydrazine treatment (b) and after pyrolysis (c).

Additionally, there is a slight trend toward higher disorder as the GO content in the nanocomposites increases. Thus, the disorder in the carbon phase (as revealed by the FWHM of the D band, Fig. 6a) increases from pure GO to GO/silica nanocomposites with low GO contents (e.g., SG0.5, SG1, SG2). This effect seems to be independent of the processing step at which the samples were investigated and it has been observed in the wet gel samples, as well as in the hydrazine-reduced, pyrolyzed and the hot-pressed materials. There is no correlation between the relative intensity of the D band (I(D)/I(G)) and the GO content (Fig. 6b). Fig. 6a shows the evolution of the FWHM of the D band upon pyrolysis of GO as well as of the GO/silica nanocomposites, indicating that the evolution

The nanocomposite powders were hot-pressed in graphite dies in argon atmosphere at 1600 ◦ C (30 MPa) to yield black monoliths (diameter of ca. 10 mm and thickness of ca. 1 mm). The hot-pressed samples exhibited a polycrystalline silica matrix consisting of ␣cristobalite (major) and small amounts of quartz, as revealed by XRD data (not shown here); whereas the GO-derived carbon phase did not crystallize. The presence of crystalline silica is confirmed by SAED (Fig. 7); however, the matrix is found to still contain some amorphous silica (Fig. 7). The SAED of the rGO phase indicate a higher ordering than it was concluded for the case of the pyrolyzed rGO/silica nanocomposite powders (Fig. 3). Obviously, some ordering of the rGO phase as compared to that of the carbon phase in the pyrolyzed samples occurred during the hot-pressing step; this was further supported by the Raman spectroscopy data of the hotpressed samples (not shown here). Fig. 8 shows the unstructured amorphous silica grains and the few-layered rGO sheets. The sharp edges and the electron transparency of the carbon-based phase is similar to the nanocomposite sample obtained upon pyrolysis. The inter-layer distance in the rGO phase was determined to be 0.35 ± 0.03 nm, which is close to the inter-layer spacing of graphite (0.34 nm [54]).

Fig. 6. FWHM (a) and I(D)/I(G) ratio (b) evolution as functions of the processing step and rGO concentration.

Please cite this article in press as: C. Hintze, et al., Facile sol–gel synthesis of reduced graphene oxide/silica nanocomposites, J Eur Ceram Soc (2015), http://dx.doi.org/10.1016/j.jeurceramsoc.2015.11.033

G Model JECS-10388; No. of Pages 8 6

ARTICLE IN PRESS C. Hintze et al. / Journal of the European Ceramic Society xxx (2015) xxx–xxx

Fig. 7. TEM micrograph of a hot-pressed rGO/silica nanocomposite. The insets depict SAED of the areas indicated by arrows.

3.4. Electrical conductivity of hot-pressed monolithic rGO/silica nanocomposites Data concerning the electrical conductivity of hot-pressed rGO/silica nanocomposite samples were obtained from impedance spectroscopy (IS). Additionally, a four-point resistivity measurement set-up was used. The evolution of the electrical conductivity with increasing the rGO content in the nanocomposites is presented in Fig. 9. As it can be seen in Fig. 9, for lower rGO contents the electrical conductivity seems not to be affected by the rGO content and starts to significantly increase only from SG5 toward higher rGO contents.

The critical volume fraction of fillers needed to obtain bulk conductivity in composites consisting of an insulating matrix and a conductive disperse phase can be numerically estimated [60]. Thus, for a completely random, three-dimensional distribution of graphene nanoplatelets,  the following formula can be applied: Vf =



(27Dt) / 4 D + Dip

3

, where Vf is the critical volume fraction,

D and t are the diameter and thickness of the particles, respectively, and Dip the inter-particle distance. Since hopping conduction is possible across thin matrix layers between conducting particles, the filler platelets are not necessarily required to be in contact with each other [39]. Assuming only rGO particles of five layers with a total thickness of 1.8 nm (according to SEM and TEM data), and a lat-

Fig. 8. TEM micrographs obtained from a hot-pressed rGO/silica nanocomposite.

Please cite this article in press as: C. Hintze, et al., Facile sol–gel synthesis of reduced graphene oxide/silica nanocomposites, J Eur Ceram Soc (2015), http://dx.doi.org/10.1016/j.jeurceramsoc.2015.11.033

G Model JECS-10388; No. of Pages 8

ARTICLE IN PRESS C. Hintze et al. / Journal of the European Ceramic Society xxx (2015) xxx–xxx

7

4. Conclusion A cheap, straight-forward sol–gel technique was employed to produce rGO/silica nanocomposite materials with high quality rGO as single carbon phase. Raman and FTIR studies indicate that the rGO phase is covalently linked to the silica matrix upon Si O C linkage, thus resulting in a very strong and stable interface. One of the particularities of the present samples is the partial crystallinity of the matrix, probably providing extra stiffness and stability not achievable with amorphous silicone-derived structures. The prepared monolithic rGO/silica nanocomposite samples were shown to exhibit an appreciable electrical conductivity at rather moderate rGO contents, which relies on the formation of a rGO percolative network, as shown by means of conductive AFM. Fig. 9. Electrical conductivity of the hot-pressed rGO/silica nanocomposite samples.

Acknowledgments The authors acknowledge financial support from the German Science Foundation (DFG) within the frame of MWN (Materials World Network) as well as from the state of Hesse, Germany (LOEWE-Zentrum AdRIA—Adaptronik: Research, Innovation, Application). C.H. gratefully acknowledges the Erasmus Mundus Fellowship for her Master Study at TU Darmstadt. The conductive AFM measurements were performed by T. Kaule and L.-O. Heim at the Center of Smart Interfaces of TU Darmstadt.

References

Fig. 10. Topographic (top) and conductive (bottom) AFM micrographs of SG10.

eral size of 1 ␮m (as indicated by AFM), a critical volume fraction of 3.81 wt% can be estimated in our nanocomposites, i.e., corresponding to samples SG10 or SG20. Conductive AFM measurement of SG10 (Fig. 10) indicates that the rGO flakes form a percolative network within the insulating silica matrix, thus providing conductive paths through the sample.

[1] P. Colombo, G. Mera, R. Riedel, G.D. Soraru, Polymer-derived ceramics: 40 Years of research and innovation in advanced ceramics, J. Am. Ceram. Soc. 93 (2010) 1805–1837. [2] G. Mera, A. Navrotsky, S. Sen, H.J. Kleebe, R. Riedel, Polymer-derived SiCN and SiOC ceramics-structure and energetics at the nanoscale, J. Mater. Chem. A 1 (2013) 3826–3836. [3] (a) K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Electric field effect in atomically thin carbon films, Science 306 (2004) 666–669; (b) K.S. Novoselov, D. Jiang, F. Schedin, T.J. Booth, V.V. Khotkevich, S.V. Morozov, A.K. Geim, Two-dimensional atomic crystals, Proc. Natl. Acad. Sci. U. S. A. 102 (2005) 10451–10453; (c) K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, M.I. Katsnelson, I.V. Grigorieva, S.V. Dubonos, A.A. Firsov, Two-dimensional gas of massless Dirac fermions in graphene, Nature 438 (2005) 197–200. [4] S. Jenkins, Quantum topology phase diagrams for molecules, clusters, and solids, Int. J. Quantum Chem. 113 (2013) 1603–1608. [5] X. Huang, X. Qi, F. Boey, H. Zhang, Graphene-based composites, Chem. Soc. Rev. 41 (2012) 666–686. [6] S. Mitra, S. Banerjee, A.D. Datta Chakravorty, Graphene composites: The materials for the future arXiv preprint arXiv 1207 2012, (1995). [7] K. Bhowmik, S. Pramanik, S.K. Medda, G. De, Covalently functionalized reduced graphene oxide by organically modified silica: a facile synthesis of electrically conducting black coatings on glass, J. Mater. Chem. 22 (2012) 24690–24697. [8] Z.H. Ni, T. Yu, Y.H. Lu, Y.Y. Wang, Y.P. Feng, Z.X. Shen, Uniaxial strain on graphene: Raman spectroscopy study and band-gap opening, ACS Nano 2 (2008) 2301–2305. [9] (a) X. Du, I. Skachko, A. Barker, E.Y. Andrei, Approaching ballistic transport in suspended graphene, Nat. Nanotechnol. 3 (2008) 491–495; (b) K. Bolotin, K. Sikes, Z. Jiang, M. Klima, G. Fudenberg, J. Hone, P. Kim, H. Stormer, Ultrahigh electron mobility in suspended graphene, Solid State Commun. 146 (2008) 351–355; (c) L.A. Ponomarenko, B.D. Belle, R. Jalil, L. Britnell, R.V. Gorbachev, A.K. Geim, K.S. Novoselov, A.H. Castro Neto, L. Eaves, M.I. Katsnelson, Field-effect control of tunneling barrier height by exploiting graphene’s low density of states, J. Appl. Phys. 113 (2013) 136502; (d) K. Lee, B. Fallahazad, H. Min, E. Tutuc, Transport gap in dual-gated graphene bilayers using oxides as dielectrics, IEEE Trans. Electron Devices 60 (2013) 103–108; (e) S. Wu, L. Mao, A.M. Jones, W. Yao, C. Zhang, X. Xu, Quantum-enhanced tunable second-order optical nonlinearity in bilayer graphene, Nano Lett. 12 (2012) 2032–2036; (f) M. Han, B. Özyilmaz, Y. Zhang, P. Kim, Energy band-gap engineering of graphene nanoribbons, Phys. Rev. Lett. 98 (2007) 206805. [10] A. Jorio, M.S. Dresselhaus, Raman Spectroscopy in Nanoscience and Nanometrology: Carbon Nanotubes, Nanographite and Graphene, Wiley-VCH, Weinheim, Chichester, 2010.

Please cite this article in press as: C. Hintze, et al., Facile sol–gel synthesis of reduced graphene oxide/silica nanocomposites, J Eur Ceram Soc (2015), http://dx.doi.org/10.1016/j.jeurceramsoc.2015.11.033

G Model JECS-10388; No. of Pages 8 8

ARTICLE IN PRESS C. Hintze et al. / Journal of the European Ceramic Society xxx (2015) xxx–xxx

[11] H. Kim, A.A. Abdala, C.W. Macosko, Graphene/polymer nanocomposites, Macromolecules 43 (2010) 6515–6530. [12] V. Singh, D. Joung, L. Zhai, S. Das, S.I. Khondaker, S. Seal, Graphene based materials: past, present and future, Prog. Mater. Sci. 56 (2011) 1178–1271. [13] X. Wan, Y. Huang, Y. Chen, Focusing on energy and optoelectronic applications: a journey for graphene and graphene oxide at large scale, Acc. Chem. Res. 45 (2012) 598–607. [14] C.K. Chua, M. Pumera, Covalent chemistry on graphene, Chem. Soc. Rev. 42 (2013) 3222–3223. [15] Y. Zhu, S. Murali, W. Cai, X. Li, J.W. Suk, J.R. Potts, R.S. Ruoff, Graphene and graphene oxide: synthesis, properties, and applications, Adv. Mater. 22 (2010) 3906–3924. [16] C. Cheng, D. Li, Solvated graphenes: an emerging class of functional soft materials, Adv. Mater. 25 (2013) 13–30. [17] H. Bai, C. Li, X. Wang, G. Shi, On the gelation of graphene oxide, J. Phys. Chem. C 115 (2011) 5545–5551. [18] S. Stankovich, D.A. Dikin, G.H.B. Dommett, K.M. Kohlhaas, E.J. Zimney, E.A. Stach, R.D. Piner, S.T. Nguyen, R.S. Ruoff, Graphene-based composite materials, Nature 44 (2006) 282–286. [19] J.T. Robinson, F.K. Perkins, E.S. Snow, Z. Wei, P.E. Sheehan, Reduced graphene oxide molecular sensors, Nano Lett. 8 (2008) 3137–3140. [20] H. Feng, R. Cheng, X. Zhao, X. Duan, J. Li, A low-temperature method to produce highly reduced graphene oxide, Nat. Commun. 4 (2013) 1539. [21] J. Lim, K. Choi, J.R. Rani, J.S. Kim, C. Lee, J. Hoon Kim, S. Chan Jun, Terahertz optical, and Raman signatures of monolayer graphene behavior in thermally reduced graphene oxide films, J. Appl. Phys. 113 (2013) 183502. [22] Z. Sun, Graphene chemistry: synthesis and modulation, in: PhD thesis, Rice University, 2011. [23] I. Jung, D.A. Dikin, R.D. Piner, R.S. Ruoff, Tunable electrical conductivity of individual graphene oxide sheets reduced at low temperatures, Nano Lett. 8 (2008) 4283–4287. [24] (a) X. Liang, S. Wang, S. Liu, X. Liu, S. Jiang, A novel octadecylsilane functionalized graphene oxide/silica composite stationary phase for high performance liquid chromatography, J. Sep. Sci. 35 (2012) 2003–2009; (b) X. Yang, X. Wang, J. Yang, J. Li, L. Wan, Functionalization of graphene using trimethoxysilanes and its reinforcement on polypropylene nanocomposites, Chem. Phys. Lett. 570 (2013) 125–131; (c) L. Guardia, F. Suárez-García, J. Paredes, P. Solís-Fernández, R. Rozada, M. Fernández-Merino, A. Martínez-Alonso, J. Tascón, Synthesis and characterization of graphene-mesoporous silica nanoparticle hybrids, Microporous Mesopororous Mater. 160 (2012) 18–24; (d) X. Zhou, T. Shi, One-pot hydrothermal synthesis of a mesoporous SiO2 –graphene hybrid with tunable surface area and pore size, Appl. Surf. Sci. 259 (2012) 566–573; (e) Y.T. Kim, J.H. Han, B.H. Hong, Y.U. Kwon, Electrochemical synthesis of CdSe quantum-dot arrays on a graphene basal plane using mesoporous silica thin-film templates, Adv. Mater. 22 (2010) 515–518. [25] J.Q. Dalagan, E.P. Enriquez, One-step synthesis of mesoporous silica–graphene composites by simultaneous hydrothermal coupling and reduction of graphene oxide, Bull. Mater. Sci. 37 (2014) 589–595. [26] Y. Zeng, Y. Zhou, L. Kong, T. Zhou, G. Shi, A novel composite of SiO2 -coated graphene oxide and molecularly imprinted polymers for electrochemical sensing dopamine, Biosens. Bioelectron. 45 (2013) 25–33. [27] S. Watcharotone, D.A. Dikin, S. Stankovich, R. Piner, I. Jung, G.H.B. Dommett, G. Evmenenko, S.E. Wu, S.F. Chen, C.P. Liu, S.T. Nguyen, R.S. Ruoff, Graphene–silica composite thin films as transparent conductors, Nano Lett. 7 (2007) 1888–1892. [28] L. Kou, C. Gao, Making silica nanoparticle-covered graphene oxide nanohybrids as general building blocks for large-area superhydrophilic coatings, Nanoscale 3 (2011) 519–528. [29] L. Hao, H. Song, L. Zhang, X. Wan, Y. Tang, Y. Lv, SiO2 /graphene composite for highly selective adsorption of Pb(II) ion, J. Colloid Interface Sci. 369 (2012) 381–387. [30] S. Hou, S. Su, M.L. Kasner, P. Shah, K. Patel, C.J. Madarang, Formation of highly stable dispersions of silane-functionalized reduced graphene oxide, Chem. Phys. Lett. 501 (2010) 68–74. [31] (a) Z. Wang, J.K. Nelson, H. Hillborg, S. Zhao, L.S. Schadler, Graphene oxide filled nanocomposite with novel electrical and dielectric properties, Adv. Mater. 24 (2012) 3134–3137; (b) R. Verdejo, M. Mar Bernal, L.J. Rosamanta, M.A. Lopez-Manchado, Graphene filled polymer nanocomposites, J. Mater. Chem. 21 (2011) 3301–3310. [32] R. Verdejo, F. Barroso-Bujans, M.A. Rodriguez-Perez, J. Antonio de Saja, M.A. Lopez-Manchado, Functionalized graphene sheet filled silicone foam nanocomposites, J. Mater. Chem. 18 (2008) 2221–2226. [33] H. Yang, F. Li, C. Shan, D. Han, Q. Zhang, L. Niu, A. Ivaska, Covalent functionalization of chemically converted graphene sheets via silane and its reinforcement, J. Mater. Chem. 19 (2009) 4632–4638.

[34] L. Chen, S. Chai, K. Liu, N. Ning, J. Gao, Q. Liu, F. Chen, Q. Fu, Enhanced epoxy/silica composites mechanical properties by introducing graphene oxide to the interface, ACS Appl. Mater. Interfaces 4 (2012) 4398–4404. [35] H.J. Salavagione, G. Martínez, M.A. Gómez, Synthesis of poly(vinyl alcohol)/reduced graphite oxide nanocomposites with improved thermal and electrical properties, J. Mater. Chem. 19 (2009) 5027–5032. [36] D.C. Marcano, D.V. Kosynkin, J.M. Berlin, A. Sinitskii, Z. Sun, A. Slesarev, L.B. Alemany, W. Lu, J.M. Tour, Improved synthesis of graphene oxide, ACS Nano 4 (2010) 4806–4814. [37] A.L. Higginbotham, D.V. Kosynkin, A. Sinitskii, Z. Sun, J.M. Tour, Lower-defect graphene oxide nanoribbons from multiwalled carbon nanotubes, ACS Nano 4 (2010) 2059–2069. [38] C. Brinker, K. Keefer, D. Schaefer, C. Ashley, Sol–gel transition in simple silicates, J. Non-Cryst. Solids 48 (1982) 47–64. [39] J.R. Potts, D.R. Dreyer, C.W. Bielawski, R.S. Ruoff, Graphene-based polymer nanocomposites, Polymer 52 (2011) 5–25. [40] D. Dhawale, R. Salunkhe, V. Jamadade, T. Gujar, C. Lokhande, An approach towards the growth of polyaniline nanograins by electrochemical route, Appl. Surf. Sci. 255 (2009) 8213–8216. [41] X. Liu, H. Zhang, Y. Ma, X. Wu, L. Meng, Y. Guo, G. Yu, Y. Liu, Graphene-coated silica as a highly efficient sorbent for residual organophosphorus pesticides in water, J. Mater. Chem. A 1 (2013) 1875–1884. [42] S. Heeg, R. Fernandez-Garcia, A. Oikonomou, F. Schedin, R. Narula, S.A. Maier, A. Vijayaraghavan, S. Reich, Polarized plasmonic enhancement by Au nano structures probed through Raman scattering of suspended graphene, Nano Lett. 13 (2013) 301–308. [43] A.C. Ferrari, D.M. Basko, Raman spectroscopy as a versatile tool for studying the properties of graphene, Nat. Nanotechnol. 8 (2013) 235–246. [44] T. Koyama, Y. Ito, K. Yoshida, M. Tsuji, H. Ago, H. Kishida, A. Nakamura, Near-infrared photoluminescence in the femtosecond time region in monolayer graphene on SiO2 , ACS Nano 3 (2013) 2335–2343. [45] Z.M. Wang, W. Wang, N. Coombs, N. Soheilnia, G.A. Ozin, Graphene oxide-periodic mesoporous silica sandwich nanocomposites with vertically oriented channels, ACS Nano 4 (2010) 7437–7450. [46] S. Kim, D. Hee Shin, C. Oh Kim, S. Seok Kang, S. Sin Joo, S.H. Choi, S. Won Hwang, C. Sone, Size-dependence of Raman scattering from graphene quantum dots: interplay between shape and thickness, Appl. Phys. Lett. 102 (2013) 053108. [47] Z.M. Wang, K. Hoshinoo, K. Shishibori, H. Kanoh, K. Ooi, Surfactant-mediated synthesis of a novel nanoporous carbon–silica composite, Chem. Mater. 15 (2003) 2926–2935. [48] T. Jiang, T. Kuila, N.H. Kim, B.C. Ku, J.H. Lee, Enhanced mechanical properties of silanized silica nanoparticle attached graphene oxide/epoxy composites, Compos. Sci. Technol. 79 (2013) 115–125. [49] F. Niu, J.M. Liu, L.M. Tao, W. Wang, W.G. Song, Nitrogen and silica co-doped graphene nanosheets for NO2 gas sensing, J. Mater. Chem. A 1 (2013) 6130–6133. [50] J. Wu, Q. Tang, H. Sun, J. Lin, H. Ao, M. Huang, Y. Huang, Conducting film from graphite oxide nanoplatelets and poly(acrylic acid) by layer-by-layer self-assembly, Langmuir 24 (2008) 4800–4805. [51] C.Y. Su, A.Y. Lu, Y. Xu, F.R. Chen, A.N. Khlobystov, L.J. Li, High-quality thin graphene films from fast electrochemical exfoliation, ACS Nano 5 (2011) 2332–2339. [52] E. Pretsch, P. Bühlmann, M. Badertscher, Structure determination of organic compounds: Tables of spectral data, Springer, Berlin, 2009. [53] G. Socrates, Infrared characteristic group frequencies: Tables and charts, John Wiley and sons, Chichester, 1994 [etc.] op. [54] W. Gao, Graphite oxide: structure, reduction and applications, in: PhD Thesis, Rice University, 2012. [55] D.A. Dikin, S. Stankovich, E.J. Zimney, R.D. Piner, G.H.B. Dommett, G. Evmenenko, S.T. Nguyen, R.S. Ruoff, Preparation and characterization of graphene oxide paper, Nature 448 (2007) 457–460. [56] J.R. Lomeda, Manipulation of carbon nanostructures for multifunctional composite materials, in: PhD Thesis, Rice University, 2010, August. [57] Z. Ni, L. Liu, Y. Wang, Z. Zheng, L.J. Li, T. Yu, Z. Shen, G-band Raman double resonance in twisted bilayer graphene: evidence of band splitting and folding, Phys. Rev. B 80 (2009) 125404. [58] T. Szabó, O. Berkesi, I. Dékány, DRIFT study of deuterium-exchanged graphite oxide, Carbon 43 (2005) 3186–3189. [59] L. Xu, J. Feng, J. Li, X. Liu, S. Jiang, Graphene oxide bonded fused-silica fiber for solid-phase microextraction-gas chromatography of polycyclic aromatic hydrocarbons in water, J. Sep. Sci. 35 (2012) 93–100. [60] J. Li, J.K. Kim, Percolation threshold of conducting polymer composites containing 3D randomly distributed graphite nanoplatelets, Compos. Sci. Technol. 67 (2007) 2114–2120.

Please cite this article in press as: C. Hintze, et al., Facile sol–gel synthesis of reduced graphene oxide/silica nanocomposites, J Eur Ceram Soc (2015), http://dx.doi.org/10.1016/j.jeurceramsoc.2015.11.033